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Summary Genetics Chapter 11
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Chapter 11: DNA replicationGenetic material is transmitted from parent to offspring and from cell to cell. For transmission to occur, the
genetic material must be copied. During this process, known as DNA replication, the original DNA strands are
used as template for the synthesis of new DNA strands.
Structural overview of DNA replication
Double helix: composed of two DNA strands, and the individual building blocks of each strand are
nucleotides. Each nucleotide contains one of the four bases: adenine, thymine, guanine, cytosine. The
double-stranded structure is held together by base stacking and hydrogen bonding between the bases in
opposite strands. A-T 2 hydrogen bonds and C-G 3 hydrogen bonds. AT-CG rules is basis for complementarity
of the base sequences in double-stranded DNA. Strand within the helix have an antiparallel alignment. This is
determined by the orientation of sugar molecules within sugar-phosphate backbone. One strand 5’ to 3’ the
complementary strand 3’ to 5’.
Existing DNA strands act as templates for the synthesis of new strands
During replication, the two complementary strands of DNA
come apart and serve as template strand, or parental strands,
for the synthesis of two new strands of DNA. After separation
individual nucleotides have access to the template strand.
Hydrogen bonding between individual nucleotides and the
template strand must obey the AT/GC rule. To complete the
replication process, a covalent bond is formed between the
phosphate of one complete and the sugar of the previous
nucleotide. Two newly made strands = daughter strands. DNA
is replicated in such a way that both copies retain the same
information, the same base sequence as in the original
molecule. The two features that allow DNA to be replicated are
its double-stranded structure and the base pairing between AT
and GC.
Three different models were proposed to describe the end result of DNA replication
Conservative model: both parental strands of DNA remain together following replication. The two
newly made daughter strands also remain together.
Semiconservative model: double-stranded DNA is half conserved. The newly made DNA contains
one parental and one daughter strand. Correct.
Dispersive model: segments of parental DNA and newly made DNA are interspersed in both strands
following replication.
, Method to distinguish newly made daughter strands and parental strands involves labelling DNA with a
heavy isotope of nitrogen. Nitrogen which is found in the bases of DNA, occurs in both heavy and light form.
They produced a population of cells only consisting of heavy DNA 15N. At the start of the experiment they
switched to a medium containing light 14N and collected samples of cells at various time points. Because the
bacteria were doubling in a medium that contained only light Nitrogen, all of the newly made DNA strands
were labelled with light nitrogen, but the original strands remained heavy. Then Meselson and Stahl
analyzed the density of the DNA by centrifugation. If one strand contained light nitrogen and the other
heavy the DNA was half-heavy and had an intermediate density -> semiconservative model.
Bacterial DNA replication: the formation of two replication forks at the origin of replication
Bacterial chromosomes contain a single origin of replication
Origin of replication = the site on the
bacterial chromosome where DNA
synthesis begins. Bacterial
chromosomes have only one. The
synthesis of new daughter strands is
initiated within the origin and
proceeds in both directions
(bidirectionally) around the bacterial
chromosome. This means that two
replication forks move in opposite
direction outward from the origin.
Replication fork = the region where the parental DNA strands have separated and new daughter strands are
being made. Eventually, the replication forks meet each other on the opposite side of the bacterial
chromosome to complete the replication process.
Replication is initiated by the binding of DnaA proteins to the origin of replication
The origin in E. coli is named oriC, for origin of chromosomal replication. Three types of DNA seuqnces are
found within oriC:
- An AT-rich region
- DnaA box sequences
- GATC methylation sites
DNA replication begins with the binding of DnaA proteins to
sequences within the origin of replication known as DnaA boxes.
When DnaA proteins are in their ATP-bound form, they bind to five
DnaA boxes in oriC to initiate DNA replication. They also bind to each
other to form a complex. Other DNA-binding proteins, such as HU
and IHF, cause the DNA to bed around the complex od the DnaA
proteins, which results in the separation of the strands at the AT-rich
region.
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